1. Field of the Invention
The present invention refers to a starting switch having a protection against dust and small particles for rotating electric machines and to a method of assembling said switch.
2. Description of the Related Art
The protection of starting switches has been studied for years by the manufacturers of several types of rotating electric machines. This is due to the fact that these switches usually operate in harsh environments, including some that may contain corrosive chemical substances and hazardous particles. Also, some environments may have high humidity levels, and the combination of these contaminant elements may accelerate the deterioration of the active components of the starting switch, thereby jeopardizing its good operation and limiting its useful life.
Typically, a rotating electric machine is used to operate a saw or similar equipment and the chipped wood and dust resulting from the cutting process usually creates an environment that is very aggressive to the switch.
In yet another case, in an environment where cotton is manufactured, small particles of cotton in suspension in the air, when put in contact with the energized parts of the switch they may cause combustion due to the occurrence of sparks in the starting switch during its operation.
Another example of a harsh environment are the nautical applications, where the presence of salt spray penetrates into the parts of the electric machine, causing premature corrosion of the conductive parts.
Another concern is the presence of insects in some applications, which insects might as well enter the equipment and settle near the conducting parts of the contact, lately impairing its operation.
Open electric machines have been used for years as they are cheaper and more compact. In this case, a solution for those environments would be the use of an encased electric machine. However, in addition to the problem of dissipating the heat generated by the machine in an encased construction, the increase in the manufacturing costs of such equipment is evident as more material is required to obtain the same power from the equipment due to the loss by heat.
The size-costs related to the increased use of copper, steel and weight are the main consequences of the use of encased machines and are good reasons for the use of open electric machines wherever the application allows, even in contaminated environments that may affect performance.
A number of the rotating electric machines used in the above-described examples have a starting capacitor or are of the split-phase type. Typically, an auxiliary winding, usually driven by a centrifugal actuator disk, is required to help start a single-phase electric machine having such a design, which winding activated until the machine reaches its working speed, after which it is necessary to switch off the auxiliary winding.
The centrifugal actuator is fixed to the shaft of the machine and is designed in such a manner that at the right time, that is, when the machine reaches its ideal rotating speed, it is axially displaced to release the contacts, thereby opening the connection and switching off the auxiliary winding. From that point on, the main winding has enough torque to fully operate the machine.
If the auxiliary winding does not act together with the starting switch and the centrifugal actuator, this is an indication of engine failure and of the need of maintenance and cleaning of the set, which causes losses and drawbacks in the operation of the machine.
Therefore, it is highly desirable a design that provides a resistant and low maintenance switching assembly, preferably an assembly that dispenses maintenance.
Several types of starting switches are similar in operation. U.S. Pat. No. 4,289,988 is an example of such starting switches. Other switches incorporate protection systems as disclosed in U.S. Pat. Nos. 2,816,199 and 4,927,988, which describe dust-proof mechanisms.
Generally speaking, a switching assembly as disclosed in U.S. Pat. Nos. 2,816,199 and 4,927,988 consists of a thin flexible plate made from a bronze-alloy or another good conductor material resistant to voltaic arcs, fixed at one end to a support having a cylindrical contact made of a silver alloy or another excellent conductor material, which when at a static condition another cylindrical contact made of a silver alloy to command, through the centrifugal actuator, the opening and closing of the contacts to start or switch off the auxiliary winding.
In more modern dust-protection systems, the gap between the contacts is insulated to keep the surfaces of the contacts clean of any dust. When these systems are compared with the ones cited as examples of the state of art, it is easy to notice the use of a flexible cylindrical casing structure, as it is the case of U.S. Pat. No. 4,927,988, which casing is also shown in other switches.